Dump vehicles, for carrying a load (object of transportation) such as excavated earth and sand to a dumping site and discharging the load by tilting the body, are used at quarries, mines, etc. When a dump vehicle is parked on irregular ground or sloping ground, the dump vehicle may fall into an unstable state due to tilting of the vehicle itself, fragility of the ground, etc. As its body is tilted to discharge the load in such a state, the barycenter of the entire vehicle rises due to the tilting of the body which causes the vehicle to become unstable and possibly overturn. There has been known a technique for preventing the overturn upon discharging of a load, in which the tilt angle of the vehicle is monitored using measurement means such as an angle meter and the tilting action of the body is restricted when the tilt angle has increased to a prescribed angle or more (see Patent Document 1).